1. Field of the Invention
The present invention relates to a photo diode and a related method for fabrication. More particularly, a photo diode for an active pixel sensor (APS) is presented, along with a related fabrication method.
2. Description of the Prior Art
Active pixel sensors are commonly used as solid image sensors. The active pixel sensors comprise of complimentary metal-oxide semiconductors (CMOS), and are also referred to as CMOS image sensors. CMOS image sensors are manufactured by traditional semiconductor manufacturers, and have lower costs and smaller sizes than regular image sensors. Therefore, applications of CMOS image sensors have been gradually replacing charge-coupled devices (CCD) over time. Furthermore, CMOS image sensors have high quantum efficiency and low read-out noise, making its usage popular with PC cameras and the digital cameras.
A general active pixel sensor comprises a plurality of active pixel sensor units. Each active pixel sensor unit further comprises a photo diode for sensing light, and three metal-oxide semiconductors. The metal-oxide semiconductors respectively serve as a reset transistor for resetting a MOS, a current source follower transistor for following a current source, and a row-select transistor for selecting a row. The photo diode sends signal data according to photo current from the photo sensor area. For example, light current serves as signal data which is generated when the photo sensor area is illuminated, and dark current is noise which is generated when the photo sensor area without light. Therefore, the photo diode utilizes the magnitude of the signal or noise to generate the signal data.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a photo diode according to the prior art. According to FIG. 1, a photo diode 100 comprises a P type substrate 102, with an oxide layer 104 made from SiO2 formed onto the P type substrate 102. The P type substrate 102 comprises a plurality of shallow trench isolations (STI) 106, a photo sensor area 108, a gate 112, source/drain extended areas 114, and source/drain areas 116. The source/drain extended areas 114 are located between the two STIs 106. The photo sensor area 108 comprises doping areas 118, 120. The doping area 118 has a low concentration of N dopant, and the doping area 120 has high concentration of N dopant, with arsenic (As) typically utilized as the dopant. The N dopant within the doping area 118 and the P type substrate form a PN junction. The PN junction and the P type substrate 102 form a depletion region to sense the current.
One of the shortcomings with the photo diode 100 according to the prior art is that the crystal grain in the surface of the photo sensor area 108 is easily damaged through a high dose of N dopant in the ion implanting process. Furthermore, in continuous contact plug manufacturing, the etching for buried contact (BC) windows easily causes damage to the surface of the doping area 120. When titanium (Ti) in the contact plug reacts with the silicon in the substrate, TiSi will be produced. The above situation can thus cause excess currents inadvertently increasing noise, and subsequently decreasing the sensitivity of the photodiode.
Furthermore, the PN junction in the photo sensor area 108 of the photo diode 100 according to the prior art is disposed within a deep area. If a short wavelength (e.g. blue light) reaches the photo diode 100, it may not be able to sufficiently penetrate the substrate, causing the PN junction of the photo diode 100 to generate an insufficient current. Therefore, the photo diode 100 possesses a low photosensitivity for short wavelength light.